In the semi-conductor chip industry it is well known that pattern transfer from a photomask (hereinafter referred to as a mask) to a substrate is accomplished by exposing the mask to a light source. During the pattern transfer process, also called the photolithographic process, patterns on the mask are projected onto the substrate, which has been treated with a photosensitive substance. This results in mask etchings being reproduced onto the substrate. Unfortunately, any foreign substance on the surface of the mask also will be reproduced on the substrate and therefore will interfere with proper pattern transfer to the substrate.
To eliminate contamination of the mask surface, a pellicle is mounted on the mask surface. A pellicle is a thin (˜1 μm) polymer film stretched across a frame that is attached to the mask. Particles deposited on the pelliclized mask fall onto the pellicle or glass backside of the mask, and therefore are several millimeters away from the features that are being imaged. With small depths-of-field, these particles will not be in focus and thus will not interfere with pattern transfer.
Referring to FIGS. 1A and 1B, a side and bottom view of a prior art mask and pellicle are illustrated. The mask 10 includes a mask pattern 12 which is formed on a substrate 14 using techniques similar to those used in wafer processing. The mask 10 also includes a pellicle 16, which includes a pellicle frame 18 attached to the substrate 14 and a transparent film 20 stretched across the pellicle frame. The pellicle 16 spans an area over the mask pattern 12 so as to prevent contamination of the mask pattern 12.
As integrated circuit designs become more complicated, it becomes increasingly important that the masks used in photolithography are accurate representations of the original design layout. Unfortunately, it is unrealistic to assume that an electron beam machine and other machines used to manufacture these masks can do so without error. Although mask makers typically repair most defects found at early inspection stages, invariably, defects are found at later inspection stages (such as after pelliclization of the mask has occurred).
Despite the many precautions which are taken to prevent damage to the mask, it still is necessary to periodically check the mask in order to assure that the pattern projected through the mask is the same as the desired pattern. Inspection is necessary to ensure the fidelity of the masks. Patterns on the masks must meet stringent criteria for size, shape, spacing, orientation, overlap, and placement of features. Defects must be repaired to prevent replication of errors across the wafers.
Generally, the later inspection stages include measuring the mask CD using an electron beam CD measurement tool. Since the pattern on the mask to be measured is behind the pellicle, the pellicle is removed before the measurement is made to increase the accuracy of the measurement.
A drawback of the present method for inspecting pelliclized masks is that before the inspection can be performed the pellicle must be removed. Pellicle removal can lead to several problems. For example, the mask can be damaged during the removal and/or reinstallation of the pellicle. Additionally, the removal and installation of the pellicle requires a certain amount of time, thereby adding to the cost of ownership of the mask. Furthermore, material costs associated with a new pellicle also are a factor in the cost of ownership of the mask.
As a result, there exists a need in the art for a method of inspecting a pelliclized mask that reduces the inspection time and material costs associated with mask inspection, thereby reducing cost of ownership of the mask.